The invention relates to a method of addressing a plasma display panel. More particularly, the invention relates to a type of panel with separate addressing and sustaining.
Plasma display panels, called hereafter PDPs, are flat-type display screens. There are two large families of PDPs, namely PDPs whose operation is of the DC type and those whose operation is of the AC type. In general, PDPs comprise two insulating tiles (or substrates), each carrying one or more arrays of electrodes and defining between them a space filled with gas. The tiles are joined together so as to define intersections between the electrodes of the said arrays. Each electrode intersection defines an elementary cell to which a gas space corresponds, which gas space is partially bounded by barriers and in which an electrical discharge occurs when the cell is activated. The electrical discharge causes an emission of UV rays in the elementary cell and phosphors deposited on the walls of the cell convert the UV rays into visible light.
In the case of AC-type PDPs, there are two types of cell architecture, one called a matrix architecture and the other called a coplanar architecture. Although these structures are different, the operation of an elementary cell is substantially the same. Each cell may be in the ignited or xe2x80x9conxe2x80x9d state or in the extinguished or xe2x80x9coffxe2x80x9d state. A cell may be maintained in one of these states by sending a succession of pulses, called sustain pulses, throughout the duration over which it is desired to maintain this state. A cell is turned on, or addressed, by sending a larger pulse, usually called an address pulse. A cell is turned off, or erased, by nullifying the charges within the cell using a damped discharge. To obtain various grey levels, use is made of the eye""s integration phenomenon by modulating the durations of the on and off states using subscans, or subframes, over the duration of display of an image.
In order to be able to achieve temporal ignition modulation of each elementary cell, two so-called addressing modes are mainly used. A first addressing mode, called xe2x80x9caddressing while displayingxe2x80x9d, consists in addressing each row of cells while sustaining the other rows of cells, the addressing taking place row by row in a shifted manner. A second addressing mode, called xe2x80x9caddressing and display separationxe2x80x9d, consists in addressing, sustaining and erasing all of the cells of the panel during three separate periods. For more details concerning these two addressing modes, a person skilled in the art may, for example, refer to U.S. Pat. Nos. 5,420,602 and 5,446,344.
FIG. 1 shows the basic time division of the xe2x80x9caddressing and display separationxe2x80x9d mode for displaying an image. The total display time Ttot of the image is 16.6 or 20 ms, depending on the country. During the display time, eight subscans SB1 to SB8 are effected so as to allow 256 grey levels per cell, each subscan making it possible for an elementary cell to be xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d for an illumination time Tec which is a multiple of a value To. Hereafter, reference will be made to an illumination weight p, where p corresponds to an integer such that Tec=p.To. The total duration of a subscan comprises an erasure time Tef, an address time Ta and the illumination time Tec specific to each subscan. The address time Ta can also be divided into n times an elementary time Tae, which corresponds to the addressing of one row. Since the sum of the illumination times Tec needed for a maximum grey level is equal to the maximum illumination time Tmax, we have the following equation: Ttot=m. (Tef+n.Tae)+Tmax, in which m represents the number of subscans. FIG. 1 corresponds to a binary decomposition of the illumination time. This binary representation has a number of drawbacks. The problem of contouring was identified a long time ago.
The contouring problem stems from the proximity of two areas whose grey levels are very close but whose illumination times are decorrelated. The worst case corresponds to a transition between the levels 127 and 128. This is because the grey level 127 corresponds to an illumination for the first seven subscans SB1 to SB7, while the level 128 corresponds to the illumination of the eighth subscan SB8. Two areas of the screen placed one beside the other, having the levels 127 and 128, are never illuminated at the same time. When the image is static and the observer""s eyes do not move over the screen, temporal integration takes place relatively well and two areas with relatively close grey levels are seen. On the other hand, when the two areas move over the screen, the integration time slot changes with screen area and is shifted from one area to another for a certain number of cells. The shift in the eye""s integration time slot from an area of level 127 to an area of 128 has the effect of integrating that the cells are off over the period of one frame, which results in the appearance of a dark contour of the area. Conversely, shifting the eye""s integration time slot from an area of level 128 to an area of level 127 has the effect of integrating that the cells are lit to the maximum over the duration of one frame, which results in the appearance of a light contour of the area (which is less perceptible than the dark area). This phenomenon is accentuated when the display works with pixels consisting of three (red, green and blue) elementary cells, since the contouring may be coloured.
The phenomenon explained occurs at all level transitions where the switched weights are completely, or largely completely, different. Switchings of high weight are more annoying than switchings of low weight because of their magnitude. The resulting effect may be perceptible to a greater or lesser extent depending on the switched weights and on their positions. Thus, the contouring effect may also occur with levels that are quite far apart (for example 63-128, but it is much less shocking for the eye as it then corresponds to a very visible level (or colour) transition.
To remedy is contouring problem, several solutions have been employed. One solution consists in xe2x80x9cbreaking upxe2x80x9d the high weights, which means adding subscans. Only the total image display time Ttot=m. (Tef+n.Tae)+Tmax remains fixed, which results in a reduction in the time Tmax (since Tef and Tae are incompressible time periods) and therefore in a reduction in the maximum brightness of the screen. It is possible to use up to 10 subscans, while still having correct brightness. With 10 subscans, the maximum illumination time Tmax is, currently, 30% of the total time, while the erasure and address time is of the order of 70%. FIG. 2 represents an example of addressing using 10 subscans SB1 to SB10, in which the high weights are broken up into two.
In order to reduce the large transitions and increase the number of subscans, without reducing the brightness of the screen, one technique consists in simultaneously scanning two successive rows for certain illumination values. The following equation can therefore be written: Ttot+m1. (Tef+n.Tae)+m2. (Tef+Tae.n/2)+Tmax. Since the erasure time Tef is negligible compared with n.Tae, the following equivalence may be written: Ttot≅m(m1+m2/2).(Tef+n.Tae)+Tmax. These simultaneous subscans reduce the address time by two and thus make it possible to add additional subscans without reducing Tmax. FIG. 3 shows an example of addressing with 11 subscans S1 to S11, the subscans S1 and S2 of which, corresponding to the shortest illumination times, are carried out on two rows at the same time so as to obtain an overall address time for these two subscans which is equal to the address time of a single subscan. If subscans common to two successive rows are carried out for the illumination weights 1, 2, 4 and 8, it is possible to obtain 12 subscans so as to eliminate the transitions of weight 64. However, the problem with this solution is the loss of resolution due to the simultaneous scanning of two rows.
With regard to the principle of subscans scanning two rows at the same time, one solution consists in the use of encoding with a rotating code or with multiple representation. FIG. 4 illustrates encoding with a rotating code using twelve subscans S1 to S12 with which the following illumination weights are associated: 1, 2, 4, 6, 10, 14, 18, 24, 32, 40, 48 and 56. One effect of the rotating code is to soften the switchings of high weight by reducing the number of switched weights during the switching of a high weight. To obtain the twelve subscans, a simultaneous scan of two rows is performed for the weights 2, 6, 14 and 24. Such a code furthermore allows multiple representation of the numbers: 34=32+2=24+10=24+6+4=18+14+2= etc. This multiple representation of the numbers makes it possible to code the grey levels present on the two scanned rows at the same time so that the weights 2, 6, 14 and 24 are identical. A person skilled in the art may refer to European Patent Application No. 0,874,349 (corresponding to U.S. patent application Ser. No. 09/061,419) for farther details about this technique. However, the effect of softening a switching of a high weight is reduced by the multiple coding which allows the number of subscans to be increased. In addition, the problem of loss of resolution remains since it is not always possible to have identical weights over the weights scanned simultaneously.
The invention proposes a novel scanning technique aimed at reducing the phenomenon of contouring. The scanning technique of the invention consists in adding at least one redundant subscan. The purpose of the redundant subscan is to place an additional, privileged, illumination time. The redundant subscan thus introduced makes it possible to have a quasi-steady illumination time independent of the grey level and therefore to minimize the effects of high-weight switching.
The subject of the invention is a method of displaying a video image on a display device during a display period, the said device comprising a plurality of cells arranged in rows and columns, in which method, during the display period:
each of the cells is illuminated in total for a time of between zero and a maximum display time corresponding to the maximum brightness of a cell for a given brightness setting;
single subscans are carried out so that the cells are xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d during a period specific to each of the said subscans;
at least one redundant subscan is carried out per group of rows so that the cells are xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d during a period specific to the said subscan; and
the sum of the periods specific to each of the single subscans and of the periods specific to the redundant subscan is greater than the maximum display time.